1. Field of the Invention
The present invention relates to a racket frame and more particularly to a tennis racket frame which is lightweight and has an excellent durability, a high rigidity, and an improved restitution performance.
2. Description of the Related Art
In recent years, the racket frame is demanded to have a light weight, a high rigidity, a high strength, and an excellent durability. The fiber reinforced resin is the most popular material for the racket frame. Normally the racket frame is formed by molding a thermosetting resin reinforced with fibers such as carbon fiber having a high strength and elastic modulus. The fiber reinforced resin containing the thermosetting resin as the matrix resin has a high rigidity and restitution performance, but is apt to generate vibrations when fiber reinforced resin is subjected to a shock, thus causing many tennis players to suffer tennis elbow frequently.
To overcome the problem, in recent years, there is proposed a racket frame composed of a fiber reinforced thermoplastic resin containing a thermoplastic resin superior in vibration-damping performance as the matrix resin thereof and a continuous fiber as the reinforcing fiber thereof. The racket frame made of the fiber-reinforced thermoplastic resin reflects high toughness of the thermoplastic resin, thus having characteristics such as a high resistance to shock and a high vibration-damping performance that cannot be attained by the conventional racket frame made of the thermosetting resin.
However, the thermoplastic resin depends on environment for its elastic modulus and strength more than the thermosetting resin. Thus in dependence on environment in which the racket frame is used, the characteristic of the thermoplastic resin such as rigidity is liable to change.
In addition, to comply with female and senior players' demands for hitting a ball a long distance with a small power, operability and restitution performance of a racket are regarded as important. Thus the racket is desired to be more and more lightweight (decrease of moment of inertia) and have a higher restitution performance.
As means for improving the restitution performance of the racket frame, the following three methods have been adopted:
(1) The weight of the racket frame is increased to increase the moment of inertia.
(2) The area of the ball-hitting face is increased.
(3) The rigidity of the racket frame in the out-of-plane direction is increased and the rigidity in the in-plane direction is decreased.
However, the method (1) reduces the operability of the racket frame and is incapable of making it lightweight. The method of (2) increases the weight of the racket frame and hence the moment of inertia. Thereby the operability decreases. The method of (3) causes alteration of a prepreg-layered construction and the sectional configuration of the racket frame. Thus if the racket frame is so constructed as to have a high elasticity, its strength decreases. If the racket frame is constructed in consideration of its strength, its weight increases.
In the racket proposed by the present applicant and disclosed in Japanese Patent Application Laid-Open No. 2003-38683, the secondary in-plane direction natural frequency is set to not less than 340 Hz nor more than 460 Hz to provide the racket with a wide sweet area. But there is room for improvement in its restitution performance.
In the tennis racket disclosed in Japanese Patent Application Laid-Open No. 2000-61004, the diameter of the string hole at its inner peripheral-side is set large to allow strings to have a large deformation amount so that the tennis racket has an improved restitution performance. However, the strength on the periphery of the string hole decreases and thus the racket frame has a low durability. If the racket frame is constructed in consideration of its strength, its weight increases.
In the tennis racket disclosed in Patent No. 2991129, the inner peripheral surface of the racket frame is concavely formed to increase the area of the sweet area. This construction enlarges the length of the periphery of the racket frame in a vertical sectional view. Thus the weight of the racket frame increases and its durability deteriorates.
In the racket disclosed in registered Japanese Utility Model No. 3090850, the inner peripheral side of the string insertion portion is formed concavely. Thereby the racket has a low rigidity in the in-plane direction and a low face stability.